Spirulina, which is a blue green algae powder, is supposed to help.
I bought some. You are supposed to disguise it in chocolate milk or food somehow. It is just like pond sludge really and it really hard to disguise. So it is sitting on the worktop
I just use sterimar spray for my nose, cetrazine when things are bad, ibuprofen when sinuses are bad at bedtime (always eat something with it) and salt water gargle for my mouth and throat.
I have mostly a problem with tree pollen earlier in the year and stay in whilst it is bad. And food - especially hazelnuts an apples and plums and more.